Bacterial chemotaxis exhibits many of the properties of sensory systems in higher organisms, and is viewed as a useful model for studying the molecular basis of sensory transduction events. The longterm objective of this work is to understand the process of chemosensory transduction in Escherichia coli at the molecular level. The components of the signaling pathway responsible for chemotactic behavior in E. coli have been identified, but their functions are not yet understood. The specific aim of this project is to explore structurefunction relationships in some of these transduction components: Tar and Tsr, inner membrane proteins that serve as chemoreceptors and signal transducers; CheA, CheW, CheY and CheZ, cytoplasmic proteins that play various roles in transmission of sensory signals between the transducers and the flagellar rotational machinery; and MotA and MotB, inner membrane proteins that are required for flagellar rotation. Plasmids carrying the structural genes for these proteins will be mutagenized in vitro and screened for various sorts of functional defects, including dominant mutations that may specifically affect interactions with other transduction components. Reversion analyses of these mutants will be done to identify the targets of the dominant effects, and to define proteinprotein interactions among the signaling elements. The DNA sequence changes in selected mutants will be determined in order to delineate functional domains within the proteins. From these studies, structurefunction models will be devised that can be tested by sitespecific mutagenesis.